Backward Black Holes Control Fate of Galaxies

Thebackward spin of a number of black holes could create mysterious jets of plasmathat control the fate of galaxies, scientists now suggest.

Atthe heart of galaxies, astronomers have routinely detected what seem to besupermassive black holes millions to billions of times the mass of our sun.Roughly a hundredth of these giants spew out jets of plasma that extend out inopposite directions.

Thesejets control how stars and other bodies form by injecting huge amounts ofenergy into the universe, playing a crucial role in the evolution of clustersof galaxies, the largeststructures in the universe. However, it remains a mystery as to how thesejets form.

Toinvestigate the origin of these powerful jets, scientists compared severaldozen galaxies whose super-massive black holes spit jets to other galaxieswhose black holes don't. All these black holes featured accretion disks ?clumps of gas and dust whirling into the maws of these dark objects. Scientistshave long known that blackholes spin.

Relyingon data collected by a Japanese space telescope dubbed Suzaku, researchersfound that jets might form right outside black holes that spin in the oppositedirection from their accretion disks. Such retrograde spin could warpspace-time in a way that forces the innermost portions of accretion disksoutward, leading to "a piling of magnetic fields that provides the forceto fuel a jet," said researcher Dan Evans at MIT?s Kavli Institute forAstrophysics and Space Research.

Thescientists looked at light from the super-hot coronas of accretion disks, madeof plasma heated by magnetic fields that lies above and below the disks,sandwiching them. These coronas generate copious amounts of X-rays that Suzakucan detect.

Afraction of light from the coronas reflects off the accretion disks, resultingin a distinct pattern called the Compton reflection hump. The majority of acorona's X-ray emissions should come from near the black hole, where matterfrom the accretion disk is falling into the black hole fastest and hottest. Assuch, the Compton reflection hump should also be most prominent there.

However,jet-emitting black holes didn't have the Compton reflection hump. This suggeststheir accretion disks had no inner regions near the black holes to reflectlight from the corona.

Thisgap in that black hole's accretion disk could result from a backward whirl.

Supercomputermodels suggest that when galaxiescollide, the merging of super-massive black holes can give the resultinggiants a decent amount of spin, and depending on the dynamics of that merger ?for instance, if galaxies of different sizes collide ?a retrograde black holecould result.

Spinningblack holes drag space-time around them, and a retrograde spin would push outthe orbit of the innermost portion of a black hole's accretion disk.

"DavidGarofalo, a generalrelativity specialist in our collaboration, has a way to describethis," Evans said. "Picture trying to get as close to the edge of aceiling fan with a pencil in your hand without hitting the fan. It's mucheasier to get close if you're co-rotating with the fan, moving the samedirection as it, as the fan creates a sucking effect. If you're moving in theopposite direction, counter-rotating with the spin of that fan, the air iseffectively pushed out at you, generating an opposing force, and you get muchfurther from that fan. The same thing happens with spinning black holes, wherethe force you feel is roughly analogous to the wind."

Inthe future, Evans said NASA's Nuclear Spectroscopic Telescope Array (NuSTAR), asatellite planned for launch in 2011, may help astronomers solve this blackhole mystery, being 10 to 50 times more sensitive that current technology.

Thescientists detailed their findings in the Feb. 10 issue of the AstrophysicalJournal.